Glow plugs have been conventionally used for, for example, pre-heating of diesel engines. A ceramic glow plug including a ceramic heater used as the heater for heating is also used as such a glow plug.
In the above ceramic glow plug, the ceramic heater includes a base formed of an insulating ceramic, and a heater element formed of a conductive ceramic and embedded in the base. The ceramic heater is held in a metal sleeve formed into a tubular shape, and the sleeve is united with a cylindrical tubular metal housing by being press-fitted into a front end portion of the housing. The housing has on its outer circumferential surface a threaded portion to be screwed into a mounting hole of an internal combustion engine.
Also, there has been known a ceramic glow plug whose sleeve has a small-diameter portion located at its rear end, and a large-diameter portion located frontward of the small-diameter portion and having a diameter larger than that of the small-diameter portion (see, for example, Patent Document 1). The small-diameter portion is press-fitted into a front end portion of the housing, and a part of the large-diameter portion and a part of the front end portion of the housing, which parts are in contact with each other, are externally welded and joined together for reinforcement.